Numerical investigation of high-frequency mechanical impact treatment of aluminum alloy thin sheet welded joint

High-frequency mechanical impact (HFMI) like ultrasonic peening (UP) exhibits a significant fatigue life enhancement of welded joints. Previous studies with regard to effects of HFMI mainly focus on welded components or structures of plate thickness greater than 5 mm. The study aims to develop a new numerical approach for predicting the surface state induced by UP process, and investigate the residual stress distribution of 3-mm 7075 aluminum alloy thin-walled welded joint. A novel model was developed to introduce the excitation amplitude of the ultrasonic horn as an input parameter and investigate the pin kinematics. A parametric study of the UP process parameters on aluminum alloy thin sheet welded joint was performed. The results of pin kinematics indicate that the pin is accelerated at the initial stage of peening and presents stochastic and high-frequency oscillations after the initial phase. For thin sheet welded joint, tensile transverse residual stresses are induced in the surface layer of the weld toe groove, which have negative effect on fatigue performance. The sensitivity of process parameters shows that relatively low intensity of peening, including moderate excitation amplitude and high treatment speed, is recommended to obtain desired residual stress and weld toe geometry, as well as high work efficiency in industrial application. The numerical results of pin kinematics and the surface state of welded joint after UP agree well with the experimental results.